It might also be useful to read a more modern retelling/sequel, perhaps a bit more expansive and accessible and given more (mathematical) 'reality' to the 'verse. For me, that's Flatterland by Ian Stewart, but there are others out there.

(Actually my first encounter with Flatland was as a meta-verse within The Incredible Umbrella by Marvin Kaye, but that meant it was initially the least understandable element, compared with the presence of Sherlock Holmes (who grasped the nature of Flatland remarkably well, and I think did his 'explaining to Watson' thing to keep us, the reader, more informed) and an entire Gilbert And Sullivan culture, compulsory operetta singing and all).

MrY wrote:But I'm looking for a universe less like ours. Not a universe based on our with a little change, really another universe.

Do you know some of them? There is a list on the net that I did not find?

The Bible. Koran. Oz. Greek mythology. The Marvel Universe. Percy Jackson. Any book about faster than light travel. The Ocean At The End of The Lane, Coraline. In fact any book by Neil Gaiman. Harry Potter. Pay attention to the physics of Hermione's purse and Ron's tent, as well as the door to Diagon Alley. Ready Player One's game space.

Perhaps the strangest I've see is in The Stars My Destination, by Bester. The story happens on 1 timeline with multiple with intersections, as that timeline crosses itself, multiple times, by a character who can leap to any when, and who has a weapon initialized by thought and will. He is given directions to escape a fire in color, taste, and other things, while suffering synesthesia. His antagonist is a woman whose vision is shifted in the spectrum, so she can see radar, while blind to visible light.

A completely different Universe would be unintelligible to you. You wouldn't have any referents. How different do you want it to be?

A really nice one is the Orthogonal series by Egan - a trilogy of sci-fi novels exploring a universe with Euclidean spacetime (rather than Minkowskian, as ours is). They're interesting in their own right, with recapitulations of modern social struggles in a pretty alien species, but they're also a brilliant and hard sci-fi exploration of what that kind of spacetime geometry means for light, relativity, and time-travel, complete with math-heavy Appendixes at the end of each book.

Another book by Egan, Incandescence, explores physical laws in our own universe, but in a very different place, in close orbit around a black hole. This provides the inhabitants with a very alien-looking set of basic physical principles, but really it's just our native relativity, seen with a much stronger and more immediate effect than we're used to in the relatively-flat spacetime we live in.